A buck-type power regulator includes an inductor-capacitor-diode network which is responsive to an unregulated dc input voltage for providing a fixed dc voltage at its output. A switch-mode power regulator includes circuitry which compares the output voltage of the regulator with a reference voltage, using the comparator error signal to appropriately switch the input supply of the regulator on and off. An illustrative method of effectuating the regulator switching is by comparing the error signal with a ramp signal which is periodic at a fixed frequency, switching the regulator on at the instant the increasing ramp exceeds the error signal, and holding the regulator on until the end of that ramp cycle. A switch-mode power regulator thereby generates an output voltage level which can be kept stable despite output load variations.
During start-up of a switch-mode power regulator, the output voltage begins at a low level, and gradually rises as the current increases through the inductor, causing the storage capacitor to charge. If no corrective action is provided, by the time the voltage reaches its normal operating level, the current through the drive transistor and inductor may reach an excessively high value, typically in the range of ten to one hundred times the normal operating value. This condition is unacceptable and can lead to damage to the switch-mode circuit and the inductor, as well as causing a serious overshoot of the voltage rail.
One method commonly used to limit the regulator input current during start-up is a cycle-by-cycle current control. This method controls the regulator input current by detecting a current through the drive transistor which exceeds a preselected threshold, and turning off the drive transistor for the remainder of that cycle. Input current to the regulator may also be limited using analog current limiting techniques. Analog limiting of the current is undesirable, as it leads to excessive dissipation in the drive transistor. Either method can be inaccurate, unless a precision current limit circuit is used. Such a circuit would require an extensive area on an integrated circuit die. Where the system is a hybrid circuit in which the drive transistor is separate from the control circuitry, such a current limiting circuit may require the inclusion of additional components.
In view of the above, it is clear that there exists a need to develop an improved apparatus for controlling the turn-on of a switch-mode power regulator than is currently known in the art.